1. Field of the Invention
This invention relates to an image heating apparatus for use in an image forming apparatus such as a copying apparatus or a printer, and particularly to an image heating apparatus for sandwiching a recording material carrying an image thereon between a pair of rollers and conveying it and heating the image.
2. Related Background Art
An image heating apparatus, for example, a fixing apparatus, used in an image forming apparatus has merits such as a short warming-up time, a small space and cost saving, and is used in a copying apparatus, a printer or the like.
In the construction of such a fixing apparatus, however, fixation is effected at a relatively high temperature, and this leads to the problem that in a system using a fixing roller (fixing rotatable member) using silicone rubber or fluorine rubber as an elastic layer, the rubber is deteriorated by the high temperature.
Also, there occurs so-called overshooting in which when a set temperature is reached after the closing of a power source switch or immediately after the completion of the copying operation, the roller is excessively heated by the time lag until heat is transferred from heating means to the surface of the roller. By this phenomenon as well, the elastic layer of the fixing roller is deteriorated.
Also, if the fixing operation is performed at a temperature higher than necessary, there is caused so-called high temperature offset which causes a toner to adhere to the fixing roller to thereby cause a bad image.
As means for suppressing overshooting, there has heretofore been proposed a method of stopping power supply to heating by conjecturing the time from the rise of the temperature of the fixing roller till the completion of warming up at the starting time of the image forming apparatus, or stopping power supply to a heating source by the temperature of the fixing roller reaching a predetermined temperature lower than the temperature at the completion of warming up, or decreasing the amount of heat imparted from the heating source to the fixing roller more than immediately after the closing of the power source switch when the temperature of the fixing roller reaches a predetermined temperature lower than the temperature at the completion of warming up.
In any of the above-described methods, however, the warming-up time is not sufficiently short.
Also, in a fixing device having a heating source only on the fixing roller side, the temperature of a pressing roller is needed to be made as high as possible to secure fixativeness, and has a great influence on fixativeness particularly in copying immediately after starting.
On the other hand, as means for suppressing the overshooting immediately after the copying operation, there have been proposed a method in which during continuous copying, power supply to the heating source is cut off depending on a predetermined number of sheets, and a method of decreasing the amount of heat imparted to the fixing roller more than at the start of continuous copying.
In the fixing device having a heating source only on the fixing roller side, however, there arises the problem that the temperature of the pressing roller drops and in the next copying operation, low temperature offset occurs.